Neuroethology of acoustic communication in crickets.
Crickets sing to attract mates, to promote copulation, and in aggressive interactions with rivals. Only males sing. A sexually receptive female, upon hearing the “calling song” -- the long-distance mate-attraction signal -- of a conspecific male walks or flies towards the sound source, a behavior known as phonotaxis. Once the male and female come into physical contact the male switches to a distinct signal, “courtship song”, which is part of a multimodal display that entices the female to mate. Males also sing “rivalry songs” during and, more typically, after winning, aggressive encounters with rivals. This article focuses on the neural mechanisms for the production, detection, and recognition of cricket songs.

Song production

Crickets sing by scraping the hardened edge (plectrum) of one fore-wing against a row of cuticular teeth (file) on the underside of the other wing . The resulting vibrations are filtered and amplified by resonant structures on the wings, resulting in the loud, musical tones that characterize cricket songs

Figure 1: A: wing specializations for generating near-pure-tone sounds; B: engagement of the plectrum of one wing with the file of the other wing. Modified with permission from Bennet-Clark and Bailey, 2002

(Bennet-Clark, 1999). Each scissoring movement of the two wings produces a single note or sound pulse, which consists of a series of harmonically related frequency components. In calling songs, the dominant frequency is usually the lowest harmonic (fundamental frequency) and, depending on the species, is generally in the range 3-8 kHz. Courtship songs of many species contain two types of note: some, generally of low amplitude, that are dominated by the same fundamental frequency as calling song; and others, generally much louder, in which a higher harmonic is dominant.

The songs of different species are distinguished both by sound frequency and by the temporal pattern, or rhythm, with which the individual notes are produced. This pattern is produced by a network of neurons in the central nervous system (a central pattern generator, or CPG) situated in the metathoracic and anterior abdominal portions of the ventral nerve cord (Schöneich and Hedwig, 2012). A command neuron situated in the brain controls the operation of the CPG (Hedwig, 2000); the song pattern is produced when the singing command neuron is active, which presumably is the result of sensory and hormonal stimulation.

Phonotaxis

Female crickets can approach a distant singing male using only his song as a cue. This was demonstrated some 100 years ago by Johann Regen (Regen, 1913), who showed that a female would approach a telephone receiver relaying the song of a male who sang in another room. Modern variations on this experiment have confirmed Regen's findings countless times. The video Media:Cricket_acoustic_communication_tex_ptaxis_lowres.mpg shows a female cricket (Gryllus texensis) walking on an air-supported styrofoam ball, in response to a model of her species' song played from a loudspeaker situated in the front left quadrant. By using computer-generated stimuli instead of the songs of actual males, researchers have learned what it is about a song that females find attractive. Females respond most reliably to artificial stimuli that match their own species' song in both sound frequency and temporal pattern. As discussed below, cricket ears are most sensitive to the dominant sound frequency of calling song, so that stimuli with other frequencies will be perceived as less intense; but there is no evidence that crickets can discriminate qualitatively between similar sound frequencies. Rather, crickets divide their auditory world into two broad frequency ranges (categorical perception), one of which is centered on the dominant frequency of calling song, and the other of which includes a wide range of high sound frequencies (Wyttenbach et al. 1996). High-frequency sensitivity is important for responsiveness to courtship song (see above) and also to detect the ultrasonic echolocation calls of hunting, insectivorous bats (Hoy et al., 1989).
The temporal structure of calling song (i.e. the durations, and spacing in time, of the individual sound pulses) varies among species, particularly those that are acoustically active at the same time and in the same place (Alexander 1961). Female responsiveness, measured as the probability and/or vigor with which they perform phonotaxis, is greatest for stimuli with temporal patterns that match those produced by conspecific males (see Hedwig and Pollack, 2007), implying the existence of temporal filters in the auditory pathway (see below).

Sound localization

Phonotaxis requires not only that females recognize their species-specific temporal pattern, but also that they localize the sound source. As in all animals, sound localization is based on comparison of acoustic cues at the two ears. For near-pure-tone sounds like cricket songs, the only possible cues for sound location are binaural differences in timing and/or intensity. Because of the small distance separating a cricket's ears, the maximum difference in time of arrival of sound is on the order of only a few dozen microseconds, a value that is probably too small to be resolved by the relatively simple nervous systems of these animals. To get around this problem, crickets have a pressure-difference auditory system in which anatomical specializations allow sound to reach both the external and internal surfaces of the eardrum. The path lengths to the two surfaces differ in a direction-dependent manner. As a result, there is direction-dependent interference between the externally- and internally-acting sounds that generates direction-dependent changes in effective stimulus amplitude at the eardrum of up to 15 dB or so (Michelsen et al., 1994). Behavioral tests show that crickets can resolve binaural differences of less than 1 dB (Schöneich and Hedwig, 2010).

Hearing

Crickets' ears are located on the front legs. The eardum (tympanal membrane) is set into vibration by sound. The physical characteristics of the tympanal membrane and associated structures are such that the amplitude of vibration is greatest for frequencies near the fundamental frequency of the species' calling song. This selective frequency tuning, which is reiterated and enhanced by the frequency sensitivity of auditory neurons, helps to filter out behaviorally irrelevant sounds, including the songs of other cricket species, thereby increasing the signal-to-noise ratio for further processing of communication signals. The inner ear contains approximately 60-70 auditory receptor neurons, each of which forms part of a multicellular mechanosensitive organelle, known as a scolopidium (Yack 2004). The mechanical coupling between the scolopidia and the eardrum is indirect and as yet poorly understood. Like the eardrum, the majority of receptor neurons are most sensitive to the fundamental frequency of the species' calling-song, although some are tuned to higher sound frequencies, including those that occur in courtship songs and in bat echlocation calls (Pollack and Faulkes, 1998; Imaizumi & Pollack, 1999).

Representation of intensity

Information about sound intensity is required not only to localize the singer (see above), but also to estimate his distance and, possibly, his “quality”; larger, presumably healthier, males sing more loudly. Sound intensity is represented by receptor neurons in three ways. First, because sensitivity varies among receptor neurons, the number of receptor neurons that respond to a sound increases with stimulus intensity. A faint sound will excite only the most sensitive receptors, whereas a louder sound will also stimulate receptors that are less sensitive, resulting in a larger population of responding neurons. Second, the firing rate of individual receptors increases with stimulus intensity. Third, the response latency (time between stimulus onset and the first spike of the response) decreases with stimulus intensity. Although latency by itself is not informative to the cricket, because it has no independent measure of stimulus onset, the difference in response latency between neurons of the two ears can code for the difference in intensity, and thus for sound direction. Behavioral tests, however, show that the left-right difference in response strength (number and firing rates of responding receptor neurons), rather than in latency, is the main cue for sound direction (Pollack, 2003; Schöneich and Hedwig, 2010).

The central auditory pathway

Auditory receptor neurons terminate in the prothoracic ganglion, where they provide synaptic input to several auditory interneurons that can be identified, based on their anatomy and physiology, as unique individual nerve cells that can be recognized in all individuals of a species, as well as between species. Although about a dozen distinct prothoracic interneurons have been identified (Cassaday and Hoy, 1977; Wohlers & Huber, 1982; Atkins & Pollack, 1987), behavioral roles are known only for the three best characterized of these. These have been variously named by different researchers, but the most commonly used designations are AN1 for (Ascending Neuron 1), AN2, and ON1 (Omega Neuron 1).

The axons of AN1 and AN2 terminate in the brain (hence their designation as ascending neurons), whereas ON1 is a local interneuron with processes confined to the prothoracic ganglion. All three neurons occur as bilateral pairs, with each member of the pair receiving excitatory input mainly from receptor neurons of one ear. The circuitry underlying early auditory processing is shown in Figure 3

AN1

is sharply tuned to sound frequencies near that of the species' calling song. It is the main route by which neurally encoded information about calling song reaches the brain. The song's temporal structure is represented by the timing of action potentials in AN1. AN1's spike train encodes a wide range of temporal patterns, although in at least one species, Teleogryllus oceanicus, species-typical rhythmic features are represented most accurately (Marsat and Pollack, 2005). The direction of the sound source is represented by the difference in firing rates of the left and right AN1s. The importance of AN1 for phonotaxis was demonstrated by manipulating its activity, via intracellular current injection, in crickets that were restrained in such a manner that they could walk on a spherical treadmill while their nervous system was exposed and mechanically stabilized such that specific neurons could be impaled with intracellular microelectrodes. When the response to sound of one of the AN1s was suppressed by injection of hyperpolarizing current, crickets walked towards the side of the non-suppressed AN1, no matter what the direction of the sound source (Schildberger and Hörner, 1988).

AN2

responds robustly to a broad range of high sound frequencies. It receives mixed excitation and inhibition at the dominant frequency of calling song (Moiseff and Hoy, 1983). As noted above, the courtship songs of some species include loud high-frequency elements, and AN2 responds strongly to these, suggesting that it might play a role in courtship-song recognition, although experiments show that its responses alone cannot underlie recognition of courtship song (Libersat et al. 1994).

Whereas the role of AN2 in intraspecific communication remains unclear, a well established function is to detect the ultrasonic echolocation calls of hunting bats and trigger evasive steering during flight. This is beyond the scope of the present article; interested readers are referred to Hoy et al., 1989.

ON1

receives excitatory input from receptor neurons of one ear and provides inhibitory output onto the AN1, AN2, and ON1 neurons that receive excitatory input from the other ear, thus increasing bilateral contrast and facilitating sound localization (Selverston et al., 1985; Faulkes and Pollack, 2000; Marsat and Pollack, 2007). ON1 is most sensitive to the dominant frequency of calling song but, unlike AN1, it also responds robustly to a broad range of high sound frequencies, including ultrasound, allowing it to increase bilateral contrast for both cricket songs and bat hunting calls. Interestingly, the accuracy with which ON1 represents the temporal structure of a stimulus varies with sound frequency. When stimulated with cricket-like sound frequency, ON1 best represents cricket-song-like temporal features, but in response to ultrasound, ON1's spike train encodes the broader range of temporal features that occur in bat echolocation calls (Marsat and Pollack, 2004).

Brain neurons

The “decision” of whether to respond to an acoustic signal takes place in the brain, and is relayed to the neural circuits controlling motor responses by descending neurons. The network of brain neurons responsible for temporal filtering is not yet fully understood, but selectivity for species-specific temporal features is evident in brain neurons that, based on anatomy, may be only two or three synapses removed from auditory receptor neurons. The dendrites of these neurons arborize in the same restricted region of neuropil as the axon terminals of AN1. Selectivity appears to be determined by the interplay of inhibitory and excitatory synaptic inputs, with excitation over-powering inhibition for stimuli with species-specific interpulse intervals (Kostarakos and Hedwig, 2012). Selectivity for the “correct” interpulse interval arises from a circuit in which excitation is delivered to a coincidence-detecting neuron along two pathways, one directly from AN1 and another that incorporates a delay that is implemented by post-inhibitory rebound following long-lasting inhibition. The duration of the inhibition, and thus the delay, matches the preferred interpulse interval; as a consequence, delayed excitation corresponding to the second (or, more generally, the nth) sound pulse of a song model arrives at the coincidence detector at the same time as the direct, AN1-mediated excitation caused by the first (or n-1th) sound pulse (Schöneich et al. 2015).How this activity is interfaced with motor circuitry is not yet clear, but available evidence for phonotaxis suggests that the efficiency of reflex-like circuits that link auditory input to motor input is modulated by commands descending from the brain. Consistent with this notion, several descending brain neurons, which relay the output of brain circuits to motor centers, also respond selectively to behaviorally effective temporal patterns (Zorović and Hedwig, 2011).

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